Downhole rod pumps, such as rotary pumps and stroke pumps, for extracting oil from an oil well are typically driven by a drive string extending down the downhole of the oil well. The drive string is a number of sections of tubes or rods connected end to end and passing down the downhole. Because the downhole can be quite deep, the drive string can be made up of numerous sections of these tubes or rods.
Progressive cavity pumps, one type of rotary pump, has a stator attached to the end of the production tubing lining the downhole and a corkscrew-shaped rotor attached to a bottom end of the drive string at the bottom of the downhole. The top end of the drive string is connected to a polish rod. The polish rod has a smooth outer surface to correspond with an inside surface of the production tubing lining the downhole and the top end of the polish rod connects to a drive unit. The drive unit rotates the polish rod and the drive string. Progressive cavity pumps are operative to pump fluid up the down hole by using the drive unit to rotate the polish rod which in turn rotates the drive string and the corkscrew-shaped rotor attached at the bottom end of the drive string. This rotation of the corkscrew-shaped rotor forces fluid up the annulus formed between the drive string and the inner surface of the production tubing, lining the downhole, and up to the ground surface.
During pumping, the downhole end of the drive string can become jammed by accumulation of debris, high viscosity of the medium being pumped, or obstructions in the downhole. When the drive string becomes jammed, the drive string at the end of the downhole stops rotating. The drive string continues to rotate a bit, twisting the drive string all the way up the downhole until the polish rod connected to the end of the drive string applies enough torsional force to the drive unit to trigger overload sensors in the drive unit. When the overload sensors trigger the drive unit to shut down, the drive unit typically includes a braking system that controllably releases torsion built up in the twisted drive string by “back spinning” the polish rod and the drive string in a controlled fashion using the braking system.
It is this twisting of the top end of the drive string while the bottom end of the drive string is stuck that causes torsional forces to build up in the drive string. Due to the fact that the drive string can be quite long, substantial torsional forces can be built up in the drive string when the bottom end of the drive string becomes jammed.
The braking system in theory should remove all this built up torsional force however it does not always release all of the torsional force stored up in the drive string. Often additional torsional force must be released by “picking up” the drive string. To “pick up” the drive string, the drive unit is disconnected from the polish rod, and the polish rod and attached drive string is pulled up the downhole a few centimeters. This releases the stuck drive string and can cause stored up torsional forced to suddenly release in an “uncontrolled” manner by the polish rod and attached drive string back spinning until the drive string is once again untwisted.
This “picking up” is typically done by attaching a device to the polish rod that can be attached to a support system, capable of lifting the drive string, and still allow the polish rod and the attached drive string to rotate. A conventional method uses a pony rod, a rod elevator and a rod hook for a device. The pony rod, rod elevator and rod hook are attached to the polish rod and the rod hook has a swivel which allows it to rotate with the polish rod and drive string, allowing the string to back spin and release the built up torsional forces. The polish rod and drive string are pulled a short distance up the downhole, attached to the swivelling hook, and any built up torsional force is released causing the drive string and polish rod to swivel on the hook.
Previously these hooks were often not symmetrical around their axis which caused “wobbling” as the drive string spun. Additionally, the way the hook attaches to the polish rod often results in a loose connection which allows the polish rod to move radially while it is attached to the hook. This “wobbling” is hard on the devices and can result in failure of the device which can cause damage to surrounding equipment and even physically injure workers nearby.
Stroke pumps are also commonly used to pump oil from an oil well. Stroke pumps use a pump attached at the end of the drive string to pump fluid up the downhole. Rather than rotating the drive string, the drive string is driven up and down inside the down hole to force fluid up the downhole to the ground surface. A drive unit and lever system is attached to the polish rod, which in turn is attached to the drive string. The lever system pushes and pulls the polish rod up and the attached drive string up and down the downhole to cause the pumping action.
However, stroke pumps can also become stuck in the downhole. If a stroke pump becomes stuck, the drive string must be stripped out of the downhole in order to remove the drive string from the downhole. However, as the drive string becomes freed by backing it off, the drive string can suddenly twist or jerk longitudinally in the downhole. This jerking or jumping can be quite violent and may damage the equipment being used or even injure people who are nearby.
U.S. Pat. No. 6,253,844 to Walker discloses a swivelling device for a downhole rod pump for releasing torsional forces on a drive string in an “uncontrolled” manner. The device uses a rod with a threaded end to connect to a mating thread on the polishing rod. This connecting of a threaded end of a rotating shaft to a corresponding threaded end of a polish rod causes the polish rod to rotate around the center of axis of the device, greatly reducing the chance that wobble will occur in the drive string as the drive string back spins to release built up torsional forces.
The device taught in U.S. Pat. No. 6,253,844 also uses a shock mechanism to allow the device to also handle longitudinal force placed on the swivelling device as it back spins or, if the device is being used with a stroke pump to dislodge a stuck drive string, absorb sudden jerks the drive string caused by the drive string coming unstuck. The shock mechanism uses a spring placed inside the housing and surrounding the rotating shaft. The shaft can therefore move longitudinally relative to the housing the spring to absorb any longitudinal motion. However, in order for the device to function, either the shaft or bearing supporting the shaft must slide relative to the housing which can prematurely wear the bearing and limit the size and type of bearings that can be used.